METHOD FOR MANUFACTURING GALVANNEALED STEEL SHEET

摘要

A galvannealed steel sheet is manufactured by preparing a hot-dip galvanized steel sheet, alloying the steel sheet, and controlling time and temperature for alloying thereof depending on Si and Al contents. The hot-dip galvanized steel sheet contains 0. 05 to 0. 30% C, 0.01 to 2.0% Si, 0.08 to 3.0% Mn, 0.003 to 0.1% P, 0 to 0. 07% S, 0.01 to 2.5% Al, 0 to 0.007% N, by mass, and the balance being Fe and inevitable impurities. Alloying time and temperature are controlled by the formula [Si + Al .gtoreq. 1.5 x 10-7 x t0-75 x(T - 465)3+ 0.117], where t is the total time (sec) of holding the sheet at 465 .degree.C or more on alloying the coating layer thereon, and T is the average temperature (.degree.C) of the sheet during the total time t (sec) of holding the steel sheet at 465.degree.C or more on alloying the coating layer thereon.